Metal detection apparatus have been known for some time and available in a number of different forms and arrangements. They serve in general for the detection of hidden metal objects imbedded or hidden in a medium as, for example, the soil. This apparatus operates by having eddy currents induced in the metal objects by the magnetic alternating field of a search coil which build up a counterfield and react on the receiver spool. These signals are received in the apparatus receiver winding along with the reaction of disturbance or noise signals and are brought out as a common indication. These signals have a shape and phase corresponding to the physical characteristics of the metal objects in which the eddy currents are induced, the characteristic differences being especially in their phase basis. These differences are often used for the categorizing of the detected objects.
Frequently, the useful signals are covered over by disturbance or noise signals, which with respect to the sum total can be much greater than the signals of the sought after objects. The soil which surrounds the object is relevant. This soil can contain mineral components which possess a magnetic permeability greater than 1. In that case, one speaks of magnetizable soil, which results in additional A.C. magnetic flow through the search coil and by which a signal is produced in the latter.
On the other hand, especially on beaches, salt water can be the medium in the neighborhood of the sought after objects. In this situation, you have a reaction of an electrically weak conducting medium on the detection coil. The described background effects result in disturbance signals of interest, in that the phase of the signals from the magnetizable earth are at 90 degrees to those signals from the electrically weak conducting medium approximately in the zero direction of the impedance plane. Disturbance signals stand, therefore, in a fixed phase relationship to the signals of different sought after objects. This relationship can be used in order to separate useful and disturbance signals from one another and to suppress the effect of unwanted signals on the detection results.
A magnetizable soil with a magnetic permeability of .mu.=1.15 generates in the receiver windings of a typical metal detection apparatus a signal voltage of approximately 50 mV. In such search apparatus, the boundary sensitivity of signal voltages for the smallest metal objects is in the order of 1 microvolt. The search signal of the smallest detectable metal body can thus be mixed in with a 50 thousand times larger disturbance signal. Should such a disturbance signal be suppressed through an arrangement of the phase selection, stability of the phase basis to a small fraction of one degree is necessary. Even if the phase direction of the search signal from the metal body differs 90.degree. from the accompanying disturbance signal, thus a small component of the disturbance signal in the direction of the search signal can be still many times greater than the latter.
An accumulated temperature change phase shift of 2-5 degrees experienced in the excitation and receiver windings, as well as in the circuit filters and amplifiers, can be compensated for by the use of stable circuit elements.